Spherical gas storage tank

Paul Boughton

In the future, the storage of energy in the form of biogas will play an increasingly important role around the globe. And WELTEC BIOPOWER has implemented a novel solution with the Hungarian partner INWATECH.

Where an investment for a biogas processing system with biogas feed-in station is not feasible, the intermediate storage of raw biogas is a smart alternative.

The example of the 1-MW plant of WELTEC BIOPOWER, which has been running in Szeged, southern Hungary, since December 2011, demonstrates the benefits of intermediate storage and its positive effect on the plant‘s overall efficiency.

What role will external gas storage tanks play for biogas plant operators in the future, and what shape will be preferred? Answers to these questions are currently available in Hungary. There, the extensive natural gas grid could also be used for biomethane storage. Prior to the feed-in, however, the raw gas would have to be processed, and this effort would not pay for all plant types. Therefore, this EU country increasingly makes use of spherical gas storage tanks, which, in combination with the digesters, provide a high storage capacity.

In Hungary, the daytime power feed-in tariff is twice as high. Therefore, biogas is collected in gas storage tanks at night in order to have the two 600-kW CHP modules running under full load at daytime. At the bottom line, large storage capacities mean a higher feed-in tariff.

Not only the external storage tank, but also the WELTEC digesters offer additional storage space: Apart from the spherical gas storage tank with its 660 cubic metres, each of the two 3,000-cubic metres stainless-steel digesters provides a gas buffer capacity of 1,016 cubic metres, thanks to the special design of the double-membrane roof.

In the two digesters, co-operator INWATECH produces together with Zöldforrás Energia, a subsidiary of the power supplier DÉMÁSZ, 440 cubic metres of raw gas an hour. Assuming an operating time of 7,000 CHP hours a year, the power plant‘s planned output will be more than 4 million kW/year. Going by past experience, the yield will be even higher in real-life use.

The generated heat will be used for the air-conditioning of the on-site office buildings. Additionally, the exhaust heat will serve the complete drying of lucernes and pellets and the heating of pigsties (300 kW): Heat will be supplied for about 20 h/day for nine months a year. The heat utilization will boost the plant‘s efficiency to more than 80 percent, and the thermal energy utilization will amount to about 9 million kWh/year. As the digesters consume about 1.2 million kWh of process heat, the surplus will be about 7.7 million kWh/year.

The operator could also have opted for other storage tank shapes for the plant. Besides the fact that the additional costs for the spherical gas storage in Szeged are covered by a higher feed-in tariff, the safety of the storage tank was a key criterion for the selection. As it had already used this storage tank type in other projects with excellent results, Hungarian partner INWATECH considered the spherical gas storage tank to be the solution of choice.

For more information, visit www.weltec-biopower.de

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